The present invention is generally directed to methods of removing a coating from a substrate. More particularly, the invention relates to the removal of coatings poor in aluminum (Al) content from a metal substrate, e.g., a superalloy component.
A variety of coatings are used to provide oxidation resistance and thermal barrier properties to metal articles, such as turbine engine components. Coatings currently used on components of gas turbine hot sections, such as blades, nozzles, combustors, and transition pieces, generally belong to one of two classes, diffusion coatings and overlay coatings. State-of-the-art diffusion coatings are generally formed to contain aluminide intermetallics, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide. Diffusion coatings are formed by depositing constituents of the coating on the component and reacting those constituents with elements from the underlying substrate of the component to form the coating by high temperature diffusion. Overlay coatings typically have the composition MCrAl(X), where M is an element from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. In contrast to diffusion coatings, overlay coatings are generally deposited intact, without reaction with the underlying substrate. During high temperature exposure in air, including the operating conditions within a gas turbine engine, the aluminum contents of these aluminum-based coatings forms a protective aluminum oxide (alumina) scale. Though as a result the aluminum content of the coating is depleted to some degree, the alumina scale advantageously inhibits further oxidation of the coating and the underlying substrate.
It is also known to form a diffusion aluminide coating in the surface of an overlay coating to increase the amount of aluminum available for oxidation, and thereby increase the oxidation resistance of the coating. For example, U.S. Published Patent Application No. US2002/0155316 to Zheng teaches diffusing aluminum into an MCrAl(X) coating containing less than ten weight percent aluminum. The resulting surface region of the MCrAl(X) coating contains aluminum at a higher concentration than the aluminum concentration in the original MCrAl(X) coating. As evident from Zheng, the purpose of the diffusion-aluminided coating is to promote the environmental resistance of a component during engine operation. As such, following the coating process, the component is placed in service with the diffusion-aluminided MCrAl(X) coating present on its surface so that the component is able to take advantage of the improved protection offered by the coating during engine operation.
When a gas turbine is serviced, the protective coatings present on various components of the turbine usually must be removed to permit inspection and possible repair of the underlying substrate. Removal of the coatings is typically carried out by immersing the components in a stripping solution. A variety of stripping techniques are currently available for removing different types of coatings from metal substrates. The techniques usually must exhibit a considerable amount of selectivity to remove only intended materials, while generally preserving the components' desired structures.
Methods have been previously described for selectively removing Al-based coatings by contacting the coating with an aqueous composition which comprises an acid having the formula HxAF6. Usually, A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga, and x is 1 to 6. These methods have generally been effective in selectively removing Al-based overlay coatings and diffusion coatings from substrate materials. Particular examples include the aqueous compositions disclosed in U.S. Pat. No. 6,833,328, U.S. Published Patent Application No. 2002/0100493, and EP 1162286. Example 2 reported in U.S. Pat. No. 6,833,328 describes the stripping of a diffusion-aluminided MCrAlY from a coupon that had been removed from a gas turbine bucket. The bucket had previously been in service on a gas turbine engine, and as such the diffusion-aluminided MCrAlY coating had been subjected to thermal exposure and thermal cycles for a considerable period of time, resulting in a protective alumina scale on the surface of the coating. Similarly, Example 1 in U.S. Published Patent Application No. 2002/0100493 reports the stripping of another diffusion-aluminided MCrAlY coating on a gas turbine bucket that had seen engine service, and therefore had a protective alumina scale (“oxide formation”) on its surface. As such, prior art such as the three documents identified above disclose the coating a component with an overlay (MCrAlX) coating, diffusion aluminiding the overlay coating, placing the component in service, and then subsequently removing the component from service and stripping its protective diffusion-aluminided overlay coating.
It has been recognized that MCrAl(X) coatings with less than about 12% Al by weight can have better high temperature (for example, in the 2000-2100° F. (about 1090-1150° C.) range) creep and stress rupture resistance than those with higher Al content (12% by weight or more), resulting in more use of MCrAl(X) coatings with less than about 12% Al by weight. These Al-poor (less than 12% Al by weight) coatings, however, are highly resistant to the selective stripping methods described above (aqueous compositions containing an acid having the formula HxAF6). Without an effective selective stripping process to remove these Al-poor coatings, non-selective methods must be relied on, such as very strong non-selective acids or aggressive mechanical methods, both of which can cause damage to the substrate. To reduce the risk of damaging the substrate during the process of coating removal, what is needed is an effective method for selectively removing Al-poor coatings from the substrate.